Bending wave panel speakers, particularly Distributed Mode panel speakers, otherwise known by the achronym “DML”, such as taught in WO 97/09842 and others to the present applicant, have the property of diffuse sound radiation resulting from complex bending wave action which beneficially provides wide directivity in all planes or directions. However in some applications a narrower directivity may be important, particularly in some axes or planes relative to others.
For public address purposes, for example for an airport concourse, the output of a loudspeaker is intended to be directed at the subjects. Maximum intelligible sound power is ideally directed over a specified range of height and over a wide horizontal area. If this narrower directivity requirement for the sound radiation in the vertical plane is not satisfactorily provided, sound power is wasted in driving the overall volume presented by the concourse and this wasted sound also degrades performance by echoing or reverberating around the space, degrading signal to noise ratio and reducing intelligibility.
Conventional piston/cone type line source speakers can achieve this to some degree, but suffer from significant interference between the arrays of piston elements at higher frequencies, which do not sum well in the acoustic space.
If the piston elements are then made smaller to address this issue, they have poorer low frequency output and power handling. If they are too large, the interference effects become dominant, spoiling the directivity performance. Compromises are therefore inevitable when using conventional piston drivers.
WO00/78090 to the present applicant describes a distributed mode bending wave panel speaker in which directivity in one plane is controlled by arranging the panel to have a modal axis and a non-modal axis orthogonal to the modal axis. The panel can support a plurality of resonant bending wave modes in the predetermined frequency range along the modal axis. The fundamental frequency of resonant bending wave modes along the non-modal axis is at least five times the fundamental frequency of the resonant bending wave modes along the modal axis. In this way, the sound emitted from the panel is anisotropic at frequencies where resonant bending wave modes along the modal axis, but not the non-modal axis, are excited.
The panel may be narrow, and of high aspect ratio and designed to operate with the intended bending wave modes dominant in the direction of the longer axis. There may be a span of vibration exciters across the minor axis to further encourage the modal dominance in the major axis. Such modes radiate over a wide range of angles relative to the long axis and hence if the panel is horizontally mounted a wide directivity is achieved in the horizontal plane. This is an advantage if such a speaker is mounted in this attitude above or below a video screen, and good area coverage may thus be provided to the audience.
Such a speaker is also intended to have wide directivity with respect to the minor axis. This is achieved because the high aspect ratio component of the invention consequently prescribes a relatively short minor axis, which radiates with naturally wide directivity at frequencies where it is modal.
However there is also a different requirement for a bending wave speaker with improved directional sound radiation, one which has particularly narrow directivity in one axis and simultaneously wide directivity in the other axis. A good application is public address.